The objective of the proposed research is to provide one of the first in-depth analyses of sexual concurrency among young adult women in the United States. These analyses will include investigations into how childhood and adolescent experiences, changes in personal circumstances during the transition to adulthood, and the relationship dynamics of intimate partnerships each contribute to this behavior. Understanding the determinants of sexual concurrency is a key step toward understanding variation in sexual risk-taking more broadly, and is especially important to consider during late adolescence and the transition to adulthood, when risk-taking behaviors and sexual experimentation are concentrated. Currently, little research is available on sexual concurrency among young adult women. Those studies that do exist have been limited by the use of cross-sectional data and data collected at infrequent intervals. Such data sources introduce recall bias that may result in underreporting; and additionally prevent scholars from investigating how concurrency changes over time. In contrast, new data from the Relationship Dynamics and Social Life (RDSL) study, featuring weekly data on the sexual activities of a racially and socioeconomically diverse, population- representative sample of young adult women, will allow the proposed study to address previously unanswered questions regarding sexual concurrency. The specific aims of this project will be to: (1) determine which childhood and adolescent experiences affect the likelihood of sexual concurrency and why (2) investigate how changes in personal circumstances such as enrolling in college or experiencing a pregnancy affect young women's changing likelihood of concurrency over time and (3) examine the relationship dynamics that contribute to young women's likelihood of concurrency. To meet these aims, the proposed study will draw on data from the RDSL study to combine time-varying information on young adults' sexual behavior, relationships, education, employment, and fertility, and static information on their childhood and adolescent experiences and on their attitudes toward sex and perceptions of sexual norms and risk. Fixed and random effects regression models will be used to assess how the probability of sexual concurrency differs both within and across women over a two and a half year period.